FIG. 4 is a block diagram for explaining the general handover operation in the conventional mobile communication systems.
This mobile communication system shown in FIG. 4 is comprised of a plurality of base stations 16, 18, 20, 22 for radio communication with radio terminal units 12, 14 such as cellular phones, a plurality of routers 24, 26 connected above each of the base stations 16-22, a gateway device 28 connected above each of the routers 24, 26, and a CN (Correspondent Node) 30 which is a communication terminal unit of a correspondent party connected through a communication network to the gateway device 28.
In this configuration, where there is an overlap zone, i.e., where radio zones overlap each other as in the case of a radio zone 32 formed by radio waves of the base station 16 and a radio zone 34 of the base station 18, the radio terminal unit 12 presently under communication with the base station 16 evaluates the condition of radio waves from the destination base station 18 and initiates a handover transaction.
First, the radio terminal unit 12 transmits a movement notification message, as indicated by an arrow Y1, through the destination base station 18, router 24, and gateway device 28 to the CN 30 of the communication correspondent. In response thereto the CN 30 transmits an acknowledgment as indicated by an arrow Y2 and the radio terminal unit receives the acknowledgment and thereafter receives data packets transmitted from the CN 30.
On the other hand, where there is no overlap zone, as in the case of a radio zone 36 of the base station 20 and a radio zone 38 of the base station 22, the radio terminal unit 14 first detects fade away of a radio link with the base station 20 where it is presently registered, and thereafter initiates a handover transaction.
First, the radio terminal unit 14 transmits a movement notification message, as indicated by an arrow Y3, through the destination base station 22, router 26, and gateway device 28 to the CN 30 of the communication correspondent. In response thereto the CN 30 transmits an acknowledgment as indicated by an arrow Y4, and the radio terminal unit receives the acknowledgment and then receives data packets transmitted from the CN 30 of the communication correspondent.
In the case where there exists an overlap zone as described above, if the handover transaction takes a long time and results in failure in handover within the overlap zone, there will occur a short break of communication because of failure in update of the path to the destination. In the case where there is no overlap zone, if the handover transaction takes a long time after the movement to the new base station 22, there will occur a short break of communication as long as the time taken. In either case, a significant element for improvement in performance is to keep the time for the handover transaction as short as possible. The time period of the handover transaction is largely dependent upon the time for detection of the new base station and the time for the update of the path to the communication correspondent.
Therefore, IETF presents the following technique of reducing the time period of the handover transaction, as shown in FIG. 5: relay routers 24a, 26a are introduced into the domain and movement under command of the relay routers 24a, 26a is supported by change of location registration in the relay routers 24a, 26a. 
The operation in this case will be described. First, as indicated by an arrow Y10, suppose the radio terminal unit 12 has moved from the edge router 16a to the edge router 18a. In this case, since the previous and new edge routers 16a, 18a both are connected to the relay router 24a, the movement is one under command of the relay router 24a and thus the radio terminal unit 12 transmits a location update request to the relay router 24a as indicated by an arrow Y11.
In response thereto, the relay router 24a performs path rerouting. Namely, the relay router 24a performs path rerouting from a path 42 of connection between the relay router 24a and the edge router 16a to a path 44 of connection between the relay router 24a and the edge router 18a. At this time, no change is made in a path 46 of connection between the CN 30 and the relay router 24a. 
Next, suppose the radio terminal unit 12 has moved from the edge router 18a to the edge router 20a as indicated by an arrow Y12. In this case, since there is the difference between the relay routers connected to the previous edge router 18a and to the new edge router 20a, i.e., between 24a and 26a, the radio terminal unit 12 notifies the CN 30 of the communication correspondent of the change of the relay router to 26a, as indicated by an arrow Y13.
In response thereto, the path from the CN 30 through the gateway device 28 to the relay router is rerouted from the path 46 of connection between the CN 30 and the relay router 24a to a path 48 of connection between the CN 30 and the relay router 26a, and a path from the relay router 26a to the radio terminal unit 12 has to be a path 50 of connection from the relay router 26a through the edge router 20a to the radio terminal unit 12. Namely, all the paths from the CN 30 to the radio terminal unit 12 need to be rerouted.
In the case of the conventional method, as described above, only when the radio terminal unit moves between the edge routers 16a, 18a under the same relay router 24a, there is no need for notification of update of location registration to the CN 30 of the communication correspondent and the time necessary for the handover transaction can be reduced thereby.